An887, ac induction Motor Fundamentals

FIGURE 12:
TORQUE-SPEED CURVE – SAME MOTOR WITH TWO DIFFERENT LOADS
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Torque
Torque
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T
T
Y
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m
ω
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T
l
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AN887
DS00887A-page 10
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2003 Microchip Technology Inc.
Constant Torque, Variable Speed Loads
The torque required by this type of load is constant
regardless of the speed. In contrast, the power is
linearly proportional to the speed. Equipment, such as
screw compressors, conveyors and feeders, have this
type of characteristic.
FIGURE 13:
CONSTANT TORQUE, 
VARIABLE SPEED LOADS
Variable Torque, Variable Speed Loads
This is most commonly found in the industry and
sometimes is known as a quadratic torque load. The
torque is the square of the speed, while the power is the
cube of the speed. This is the typical torque-speed
characteristic of a fan or a pump.
FIGURE 14:
VARIABLE TORQUE, 
VARIABLE SPEED LOADS
Constant Power Loads 
This type of load is rare but is sometimes found in the
industry. The power remains constant while the torque
varies. The torque is inversely proportional to the
speed, which theoretically means infinite torque at zero
speed and zero torque at infinite speed. In practice,
there is always a finite value to the breakaway torque
required. This type of load is characteristic of the trac-
tion drives, which require high torque at low speeds for
the initial acceleration and then a much reduced torque
when at running speed. 
FIGURE 15:
CONSTANT POWER 
LOADS
Constant Power, Constant Torque Loads 
This is common in the paper industry. In this type of
load, as speed increases, the torque is constant with
the power linearly increasing. When the torque starts to
decrease, the power then remains constant.